Power Supplies based upon the LLC Power Conversion topology can offer high switching frequency and excellent full power efficiency due to the inherent Zero Voltage Switching offered by this topology. For this reason, the LLC power conversion topology is used in a wide range of applications including digital televisions. The main drawback of LLC converters in this type of application is their poor light load efficiency.
Operating the LLC converter in Burst mode allows us to greatly improve the effective light load efficiency that can be achieved. In Burst mode the LLC converter operates at relatively high power for a short period of time while charging the output capacitor with energy. Between Bursts the LLC converter is turned OFF and the load is supplied from the output capacitor. Overall efficiency is greatly improved because while it is operating the LLC is delivering relatively high power, with good efficiency, and between bursts the LLC is switched OFF so it does not incur the normal switching power losses.
Burst mode operation with a primary side controller is well known for topologies in which the Optical Feedback signal from the secondary side is used to control directly the power delivery. An example of such a converter topology would be a peak current mode controlled DCM Flyback converter. In this topology the Feedback signal is used to control the peak current (hence the energy stored) in the magnetising inductor at the end of the ON period of the switch. This energy level will all (apart from some efficiency losses) be transferred to the output during the remainder of the switching cycle. There is therefore a direct link between the Feedback signal level and the energy delivered to the load. Burst operation can therefore be achieved simply by setting two thresholds on the Feedback signal, at the desired corresponding load power level, such that when the Feedback calls for a low power the power switching is disabled, and when the Feedback calls for a high power level the power switching is re-started. Provided some hysteresis band exists between the turn-on and turn-off thresholds the Burst operation will switch cleanly between the two states. The mark space ratio achieved will depend upon the current level of load power versus the position of the turn-on and turn-off thresholds. The Burst frequency achieved and output ripple voltage will depend upon the size of the output capacitor, the control loop response, and the hysteresis level between the turn-on and turn-off thresholds. The power delivered during a burst period will average out to the mid-point between the two hysteresis thresholds.
For a topology that delivers power proportional to Optical Feedback signal, such as the DCM Flyback Converter, use of the described Burst mode control approach is widely known and used.
An LLC converter has a control characteristic in which the Optical Feedback characteristic modifies LLC switching frequency, which in turn defines the voltage gain between input and output of the converter. This means that the level of Optical Feedback signal is primarily a function of input to output voltage ratio rather than being related to load power. This voltage gain feedback characteristic, or more particularly the lack of a delivered power versus feedback characteristic, prevents the use of the well known Burst control method described above.